You are Here: Home >< Maths

# Circle Tangent questions. Watch

1. I am having a fair bit of trouble getting the correct answers for a few questions in my math book

Find the equations of the tangents to the circle x^2+y^2+6x-2y-15=0 that are perpendicular to the line 4x+3y+5=0

Find the equations of the tangent fromt he given point to the given circle.

Point (0,0) Circle x^2-y^2+4x+2y+4=0

Tried to get the answer for these for the last half an hour but failed to do so.
2. For the first one you are looking for a line

You know the gradient of the line but not the intercept so you will still have +c in the equation

Then you substitute that line into the circle equation ... then you will have a quadratic in x

You are looking for a single solution so the discriminant = 0

For the other one the meted is basically the same but in your equation of a line you know c but not m
3. first find the general form of the perpandicular to the circle so you have ax+by=c.
rearrange for y= whatever
substitute y into your circle and you want the discriminant to be equal to 0, so that for the x there in only once y possible (the discriminant should be a quadractic in c so solve this equation)

for this second one find the formula of the line first and you should get y=kx.
substitute this in to find the two possible values of k using a similar method
4. Got the second question still having trouble on the 1st.
5. (Original post by Sgany)
No

me neither

is the question correct
6. (Original post by TenOfThem)
No

me neither

is the question correct
Probably not the maths book is pretty crap and full of mistakes I will talk to my teacher tomorrow about it.
7. The point (0,0) doesn't lie on the circle.

EDIT: Whoops, wrong circle.
EDIT2: Well the second one isn't even a circle, it's a hyperbola?
EDIT3: Still doesn't lie on it even if it was a circle...
8. (Original post by Sgany)
I am having a fair bit of trouble getting the correct answers for a few questions in my math book

Find the equations of the tangents to the circle x^2+y^2+6x-2y-15=0 that are perpendicular to the line 4x+3y+5=0
Is line 4x+3y+5=0 a tangent to the circle? If so then a line perpendicular to it would go through the centre of the circle. You can use the equation of the circle to find that using completing the square. Then you just need to work out the gradient of the tangent to the circle and insert it all into y-y1=m(x-x1).
Is this what you're looking for?

Updated: April 17, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### Stuck for things to do this summer?

Come and get some inspiration.

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams